Phosphors which can be efficiently excited by ultraviolet, blue or green primary radiation and have an efficient emission in the blue, green, yellow, red or deep-red spectral range are of very great interest for the production of white and colored conversion LEDs. Conversion LEDs are used for many applications, for example for general lighting, display backlighting, signage, display panels, in automobiles and in numerous further consumer products. Conversion LEDs for the backlighting of display elements, such as displays, for example, differ greatly from conversion LEDs for general lighting. The requirements made of conversion LEDs for general lighting consist, in particular, in a high luminous efficiency combined with a high efficiency, a high color rendering index and a color temperature of less than 3500 K. Conversion LEDs for the backlighting of display elements require, in particular, phosphors having narrowband emission in the blue, green and red spectral range in order to cover the widest possible color space. Moreover, there is great demand for colored conversion LEDs which render colors adapted to consumer desires (so-called “color on demand” applications).
Previous white-emitting conversion LEDs for general lighting and backlighting use a semiconductor chip which emits a blue primary radiation, and a red and green phosphor. What is disadvantageous about this solution is that the epitaxially grown semiconductor chips, based for example on GaN or InGaN, can have fluctuations in the peak wavelength of the emitted primary radiation. This leads to fluctuations in the white overall radiation, such as a change in the color locus and the color rendering, since the primary radiation contributes the blue portion to the overall radiation. This is problematic particularly when a plurality of semiconductor chips are used in a device.
In order to avoid fluctuations, the semiconductor chips are sorted in accordance with their color loci (“binning”). The narrower the tolerances set with regard to the wavelength of the emitted primary radiation, the higher the quality of conversion LEDs which consist of more than one semiconductor chip. However, even after sorting with narrow tolerances, the peak wavelength of the semiconductor chips can change significantly in the case of variable operating temperatures and forward currents. In general lighting and other applications, this can lead to a change in the optical properties, such as the color locus and the color temperature.
In the backlighting of display elements, such as displays in televisions, computer monitors, tablets and smartphones, manufacturers endeavor to render the colors in a vivid and lifelike way, since this is very attractive to consumers. The backlighting of display elements therefore requires light sources having very narrowband emissions, that is to say a small full width at half maximum, in the green, blue and red spectral range in order to cover the widest possible color space. As light sources for backlighting applications, predominantly a blue-emitting semiconductor chip is combined with a phosphor having a peak wavelength in the green spectral range and a phosphor having a peak wavelength in the red spectral range.
Conversion LEDs for backlighting applications conventionally use as green phosphor, for example, an yttrium aluminum garnet, a lutetium aluminum garnet or a β-SiAlON (Si6-zAlzO2N8-z:RE or Si6-zAlzOyN8-y:REz where RE=rare earth metal). However, yttrium aluminum garnet has an emission peak having a large full width at half maximum, such that, as a result of considerable filter losses, the achievable color space is restricted and the efficiency is also reduced. β-SiAlON with a full width at half maximum of less than 60 nm has a narrowband emission in the green spectral range which leads to a more saturated green rendering than with a garnet phosphor. However, β-SiAlONs lack a good internal and external quantum efficiency, which makes the entire backlighting not very efficient. Furthermore, the production of these phosphors requires very high temperatures and expensive equipment. The production of the phosphor is thus very expensive and thus so is the production of conversion LEDs comprising said phosphor.
Quantum dots, on account of their very narrowband emission, are also used for converting primary radiation for backlighting applications. However, quantum dots are very unstable. Moreover, most commercially available quantum dots comprise harmful elements such as Hg or Cd, the concentration of which is limited in commercial electrical and electronic devices under the RoHS regulations (“reduction of hazardous substances”, EU Directive 2011/65/EU).
Known blue-green to green phosphors for conversion LEDs are for example the phosphors Ca8Mg(SiO4)4Cl2:Eu, (Sr,Ba)2SiO4:Eu and Lu3(Al,Ga)5O12:Ce. However, conversion LEDs comprising these phosphors have inadequate color purity and cannot achieve specific color loci, for which reason they are not appropriate for many “color on demand” applications.